You can apply whatever meaningful data you want for the key/value pair. As an example: if your site marketed hotel rooms and you have a page which features information on San Diego. You could set that page’s ViewContent with a ‘Location’:’San Diego’ key value.
This information is targetable within Facebook’s Create Audience control panel by selecting Custom Combination from the Website Traffic dropdown, then include the Event ‘ViewContent’ or whatever event you are passing data for. Once that particular event receives traffic with the key/value pairs you define, you will be able to select the key name you define as a parameter to the ViewContent event.
We are working hard on a more robust website including more in-depth walkthroughs based on your questions, so please keep them coming!